Processes for the oligomerization of lighter olefins to produce C6 and higher carbon number olefins are well known. Oligomerization processes can be used to produce plasticizer components from propylene. Additionally, oligomerization processes have been long employed to produce good quality motor fuel from butylene. Such oligomerization processes are also referred to as catalytic condensation and polymerization with the resulting motor fuel often referred to as polymer gasoline. Methods have always been sought to improve the octane number of the gasoline boiling range oligomerization products. In addition, the oligomerization process is also susceptible to catalyst fouling from the condensation of heavy oligomers into coke that covers the catalyst.
Another process that has met the continuing demand for the conversion of light hydrocarbons into high octane motor fuels was the alkylation of isobutane with propylene, butenes and amylenes using a hydrofluoric acid (HF) catalyst, commonly referred to as HF alkylation. The HF process has provided a highly successful method for the production of high octane motor fuels.
A number of arrangements are known for using oligomerization in combination with other processes such as saturation and dehydrogenation as substitutes for acid catalyzed isomerization alkylation. Patents disclosing the dehydrogenation of light paraffin stream with oligomerization of the dehydrogenation effluent include U.S. Pat. Nos. 4,393,259 B1, 5,049,360 B1, 4,749,820 B1, 4,304,948 B1 and 2,526,966 B1.
In the oligomerization method of the indirect alkylation process set forth in, for example, U.S. Pat. No. 5,990,367 B1, lighter aliphatic olefins such as C3 or C4 are contacted with a solid phosphoric acid catalyst in the presence of a higher paraffin diluent such as cyclohexane or octane. The presence of the paraffin diluent is believed to promote the oligomerization in the liquid phase to yield predominantly dimerized butylene or trimerized propylene oligomers such as C8 and C9 olefins. The higher aliphatic olefins can be saturated to provide fuel or plasticizer components.
In an alternative form of the indirect alkylation process, an ionic exchange resin catalyst oligomerizes light olefins to produce oligomers such as C8 olefins. In such processes, the oligomerization zone can be preceded by a dehydrogenation zone to convert paraffinic feed into olefinic feed and/or succeeded by a hydrogenation zone to convert heavy oligomeric olefins into heavy alkanes that can be blended with gasoline stock. U.S. Pat. No. 4,313,016 B1 discloses a heat exchanged oligomerization reactor that contains a cationic exchange resin catalyst. C4 olefins contacted with the resin catalyst oligomerize to C4 oligomers. This patent discloses that water or methanol may be present in small amounts insufficient to form an entrained second phase to serve as a catalyst modifier.
Modern oligomerization processes often include an oxygenate such as tert-butyl alcohol (TBA) and/or sec-butyl alcohol (SBA) in the feed for modifying the catalyst to maintain desired product selectivity. The modifier does not participate in the oligomerization reaction. References disclosing resin catalyzed oligomerization in the presence of an oxygenate modifier include U.S. Pat. No. 5,877,372 B1 and EP 994 088 A1. TBA and SBA have become the resin catalyst modifier of preference.
It is highly desirable to operate the oligomerization reaction under plug flow conditions to assure uniform conversion along the reaction front. Maintenance of plug flow conditions assures a tighter product distribution. Without plug flow conditions, channeling and even recirculation can result. In “channeling”, segments of the reaction front move downwardly more quickly than other segments of the reaction front causing bypassing of downstream product fluid by the upstream reactor fluid. This flow instability is also called “fingering” and is a result of the fluid wanting to achieve a lower energy state. “Recirculation” involves swirling of the reactants against the direction of flow. Channeling can cause underconversion and overconversion of reactants to product; whereas, recirculation can have the same effect but to a greater degree. Overconversion can generate even greater temperatures than desired for the oligomerization reaction to proceed and can cause the catalyst to degrade by deposition of carbon particles on the catalyst which is a phenomenon known as “coking”. These effects operate to spread the product distribution away from desired products, thereby diminishing product value and consistency. Resin catalyst has a relatively low range of thermal stability. Hence, overconversion can generate reaction temperatures that exceed the range of thermal stability for resin catalyst and cause destruction of the catalyst.
It was originally thought that a downflow reactor scheme would provide sufficient reaction front stability to operate under plug flow conditions. Pilot plant studies did not alert to the fact that plug flow could not be maintained under downflow oligomerization conditions. Modeling was conducted to study the stability of the reaction front under oligomerization conditions. The study revealed not only that downflow aliphatic oligomerization would be unstable, but that it would be far less stable than anticipated. Surprisingly, the modeling study revealed that downflow was so unstable that channeling and even recirculation of reactants could take place under certain conditions.
The density of the liquid mixture in the aliphatic oligomerization reaction decreases proportionally with the progress of the oligomerization. The relatively high heat of reaction from oligomerization generates very high temperatures causing the reaction products to be less dense and more buoyant relative to the reactants even though the higher aliphatic olefin products are more dense than the lower aliphatic olefin reactants at equivalent conditions. The higher temperature effects a greater reduction in density than the composition change increases the density of the products. The viscosity of the liquid mixture in the oligomerization also decreases proportionally with progress of the oligomerization, but the effect of viscosity on stability is much less prominent than is the effect of density. Flow instability occurs when the denser inlet fluid bypasses the less dense product fluid during operation in downflow.
Upflow reactors with and without fixed catalyst beds are disclosed in the art. U.S. Pat. No. 5,789,640 B1 discloses an upflow fluidized bed system using solid acid catalysts. U.S. Pat. No. 4,255,352 B1 discloses upflow through a series of tank reactors to react an olefinic hydrocarbon and an olefinically unsaturated nitrile in the presence of a diluent predominantly comprising water to produce unsaturated dinitriles. The latter patent discloses the use of promoters which it defines to include catalysts without discussion of fixing the catalyst bed. U.S. Pat. No. 6,013,845 B1 discloses producing bisphenol from dimethyl ketone and phenol in a fluidized catalyst bed. Backmixing of catalyst and the reactor feed is minimized by packing the bed with randomly oriented packing.
Both U.S. Pat. Nos. 3,560,167 B1 and 4,801,432 B1 disclose upflow reactors with fixed catalyst beds. Both reactors are equipped for at least one gaseous reactant, although the reactions take place partially in the liquid phase, and mechanical hold-down structures are required to maintain the stability of the catalyst bed.
U.S. Pat. Nos. 4,695,665 B1, 4,051,191 B1 and 4,343,957 B1 disclose upflow processes for the production of cumene using solid phosphoric acid in fixed catalyst beds. The advisability of using an upflow scheme for an oligomerization reaction of aliphatic olefins to obtain plug flow conditions is not disclosed, nor is there any indication of the extent of the instability of an aliphatic oligomerization reaction proceeding in downflow mode.
It is an object of this invention to improve the plug flow stability and product distribution of an aliphatic olefin oligomerization reaction by operating the reaction in an upflow mode.